1. Field of the Invention
The present invention relates generally to a hammer drill. More particularly, the present invention relates to a hammer drill having a rotation transmission mechanism which is provided between a motor and a tool bit attached to the top of a drill housing and which rotates the bit, and having a impact transmission mechanism provided also between the motor and the tool bit for transmitting hammer blows to the tool bit.
2. Description of the Related Art
It is a common practice in the art to which the present invention pertains to provide a change-over device for the rotation transmission mechanism and for the impact transmission mechanism for switching between transmission and disconnection of drive power from the motor, thus changing the operation mode of the hammer drill. The Applicant disclosed in Japan Published Unexamined Patent Application No. 9-57650 a large hammer drill which incorporates a crank mechanism, the contents of which are incorporated herein by reference. The hammer drill includes a sleeve which functions as a first change-over device for selectively transmitting rotation from the motor to the tool bit when slid to one position and disconnecting the motor rotation when slid to another position. The hammer drill additionally includes a link which functions as a second change-over device for selectively transmitting reciprocating motion of the piston to the tool bit when slid to one position and disconnecting the reciprocating motion when slid to another position. Moreover, a rotary selector knob is provided in this tool for allowing the operator to simultaneously select a combination of the slide positions of the two change-over devices, i.e., one of the three possible operation modes of the hammer drill. When a rotation-plus-hammer mode is selected, both rotation and hammer blows are transmitted to the tool holder. In a hammer-only mode, only hammer blows are transmitted to the tool holder. In a neutral mode, the tool bit is manually freely rotatable in either direction, thus allowing the operator to change the rotational angle of the tool bit.
From a viewpoint of convenience, it would be preferable to have such a large hammer drill with a crank mechanism as described above be able to operate in a rotation-only mode in addition to a rotation-plus-hammer mode and a hammer-only mode. The structure of the selector knob renders implementation of a rotation-only mode in the hammer drill very difficult. Significant changes in the design and thus increase in the manufacturing cost would be inevitable if these three modes are to be realized without altering the basic structures of the impact transmission mechanism and the rotation transmission mechanism. One possible means to achieve this goal is to provide two separate operating members for independently operating a sleeve provided for transmission of rotation separate and a link provided for transmission of hammer blows. One drawback of this configuration is that since each operating member has a position in which rotation or hammer blows are disconnected (an "off" state), the operator may unintentionally and inconveniently place the hammer drill in an off-off state, in which neither rotation nor hammer blows are transmitted to the tool bit, thus rendering the tool inoperative.